There are several ways to network 2 PCs. The simplest is to install an Ethernet network card in each PC, and connect the two with a length of crossover Ethernet cable. Then simply run the network setup wizard if you have W98se or later. This is known as ICS, or Internet Connection Sharing. Both PCs will share a single modem.
click here for a simple guide.
Another method, far less popular in the UK, is to use your existing home telephone wiring to act as a network. This is known as HPNA. However, read the second article on the page at click here and you will see a few reasons why it has never really taken off.